Pliocene diatom and sponge spicule oxygen isotope ratios from the Bering Sea: isotopic offsets and future directions

Abstract. Oxygen isotope analyses of different size fractions of Pliocene diatoms (δ¹⁸O_diatom) from the Bering Sea show no evidence of an isotope offset and support the use of bulk diatom species samples for palaeoceanographic reconstructions. Additional samples containing concentrations of sponge spicules produce δ¹⁸O values several per mille (‰) lower than δ¹⁸O_diatom with a calculated mean offset of 3.9‰ ± 1.5. This difference is significantly greater than modern-day variations in water δ¹⁸O through the regional water column. Despite the potential for oxygen isotope disequilibrium within δ¹⁸O_sponge, there appears to be some similarity between δ¹⁸O_sponge and a global stacked benthic δ¹⁸O_foram record. This highlights the potential for δ¹⁸O_sponge in palaeoenvironmental research at sites where carbonates are not readily preserved.